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Role of Fungi

Fungi play a key role in relation to plant roots. They become attached to roots, and their hyphae spread out over a wider area. These fungi are called Mycorrhiza (Greek for 'fungi' & 'root').  They are becoming a quite trendy garden product. Buy a packet and add to your plants to make them grow better, the blurb says. But is it that simple? 

These fungi are not so much an addition but utterly crucial for many plants. Virtually all trees depend on them although heathers seem to have a different group of fungi. Brassicas have no such root fungal relationships. 

There are two main groups of mycorrhiza, those that grow from inside the root called arbuscular (AM)
Arbuscular Mycorrhizapaired arbuscules in flax root X200
and those that remain on the surface (EM). What is quite surprising is that the internal (arbuscular) forms date to way back, 300-400 mill years ago, while the ectopic forms appeared much later - around 100mya.

These fungi live by the energy produced by the plants, with their sugars passed to the fungi. These sugars are thus carbon transfers from plant to fungi, and ultimately the soil. Springtails appear to be the main feeders of these fungi - eating their debris but also the debris of the plant roots and root hairs. It is this process that enables the soil to become a 'carbon sink'. So the fungi and springtails play a vital role in this. By eating the fungal debris, the springtails are bringing their carbon into the soil. These fungi thus have three key functions - bring nutrients in to plants, take sugars out, and - on death - pass the embedded carbon of those sugars into the soil.

In exchange these fungi absorb various elements from the soil that are then transferred to the plant, providing it with essential minerals (eg phosphate see..). But it is not just minerals they provide as these fungi also move nitrogen into the roots. We tend to think of nitrogen forming bacteria, but it seems that these fungi will have been doing a similar job for several hundred million years before  before nitrogen fixing bacteria emerged - around 100mya

It is not clear whether these fungi are quite specific to particular plant species or whether they are generalist - thus explaining their widespread 'success'. It may be that we have a lot of discovering to do..A few years ago, a dozen sorts were found in tree roots - Glomus genera most prolific, but some years later 37 groups were found. It may be that as we use DNA printing more, we will find more variations.

Springtails clearly have major role relation with these fungi. When we say they eat it and debris dont realise just how significant that  is. C-labelling found that carbon produced in plant ends up in mycorrhiza and springtails - but not in bacteria acari or nematodes. Hedgar 2010. See onychurid for role of springtails in moving the fungal spores around..

Taxonomic organization of the phylum Glomeromycota. Fungi in this phylum are known to form arbuscular mycorrhiza, and so far three classes, five orders, 14 families and 29 genera have been described. Sensu lato, spore formation in 10 of the arbuscular mycorrhiza-forming genera is exclusively glomoid,

Glomoid spores almost lacking in pigmentation, and with a soft, pliable nature resulting in wrinkling and resistance to fracturing when crushed. Acaulosporoid spores formed in the neck of a hyaline to subhyaline to whitish sporiferous saccule, often but not exclusively on a short pedicel resulting in a glomoid appearance once detached from the collapsed saccule. Presumably this is one of mechanism of survival as there will be periodic crushing/compaction, yet soils respond.

Different hyphae more edible than other Fundamental p42/3.  There can be several sorts in one root. This implies they would be capable of responding to different temperatures/humidities etc and at different times. Would seem they change over season, as more carbon is stored later in the year..up to 5X more activity at end of summer than start - in forests Redeker 2002. At times the hyphae can be fine but where there is humus they are brown indicting some lignin protection.

These figures fit - albeit in v general terms, with my predictions of the evolution of the soil. Again it shows that there is not one factor on soil formation but a multitude working together, of with these root fungi play a crucial role. Now we also value them for their potential for holding carbon n the soil and thus mitigating global warming.

Glomulin is root exudate and is 'the glue' of the soil..do fungi eat that? Glomalin is a glycoprotein produced abundantly on hyphae and spores of arbuscular mycorrhizal (AM) fungi in soil and in roots. Glomalin was discovered in 1996 by Sara F. Wright, a scientist at the USDA Agricultural Research Service.[1] The name comes from Glomales, an order of fungi.[2] Glomalin eluded detection until 1996 because, “It requires an unusual effort to dislodge glomalin for study: a bath in citrate combined with heating at 250 F (121 C) for at least an hour.... No other soil glue found to date required anything as drastic as this.” - Sara Wright who says: We’ve seen glomalin on the outside of hyphae, and we believe this is how the hyphae seal themselves so they can carry water and nutrients. 

May also be what gives fungi the rigidity they need to span the air spaces between soil particles.” Glomalin takes 7–42 years to biodegrade and is thought to contribute up to 30 percent of the soil carbon where mychorizal fungi is present. The highest levels of glomalin were found in volcanic soils of Hawaii and Japan (Look into this link, as volcanoes key part of the soil evolution plot.)

When the mycorrhizal fungi build their highways, they puncture the soil and create pockets of space for additional water to be stored. Healthy soil can hold up to 20 times its weight in water, and the highways that mycorrhizal fungi grow provide water security for plants during droughts. 

Glomalin-related soil proteins (GRSPs), along with humic acid, are a significant component of soil organic matter and act to bind mineral particles together, improving soil quality.[1][3] Glomalin has been investigated for its carbon and nitrogen storing properties, including as a potential method of carbon sequestration.[5][8] May well improve soil aggregate stability and decrease soil erosion but dont know how.

Who eats the glomalin? Presumably ..springtails. Does glomalin hold your farm together?
Diverse fungi species help plants access a variety of nutrients, and this nutrient exchange keeps our plants healthy and nutrient-rich. Monocropping, an industrial agriculture practice supported by agricultural subsidies, does not support mycorrhizal fungi diversity, and the nutrient density of our fruits and vegetables is suffering. The lack of biodiversity above ground affects the biodiversity below ground...
My thesis needs to be revisited as I did do 'biodiversity'..40 years ago!

Local climate sets the stage for the wood wide web. Reports in Nature found in cool temperate and boreal forests, where wood and organic matter decay slowly, network-building EM fungi rule. About four in five trees in these regions associate with these fungi suggesting the webs found in local studies indeed permeate the soils of North America, Europe, and Asia.

By contrast, in the warmer tropics where wood and organic matter decay quickly, AM fungi dominate. These fungi form smaller webs and do less intertree swapping, meaning the tropical wood wide web is likely more localized. About 90% of all tree species associate with AM fungi; the vast majority are clustered in the hyperdiverse tropics. Nitrogen fixers were most abundant in hot, dry places such as the desert of the U.S. Southwest.